Transcranial Magnetic Stimulation (TMS) of the brain has been employed in a limited way to treat depression refractory to the administration of drugs. The number of treatable conditions may significantly increase as the depth of the target increases. Systems for targeting neural structures at depth (e.g., Schneider and Mishelevich, U.S. patent application Ser. No. 10/821,807, now U.S. Pat. No. 7,520,848, and Mishelevich and Schneider, U.S. patent application Ser. No. 11/429,504), now U.S. Pat. No. 8,052,591 may include multiple electromagnets, the firing of which must be coordinated. TMS stimulation of deep targets would potentially permit treatment of a variety of conditions such as chronic pain, addiction, obesity, depression, Alzheimer's disease, and Parkinson's disease. Conventional rTMS (repetitive transcranial magnetic stimulation) is capable of effectively stimulating only the outer cortical layer of the brain, and treats depression indirectly, by stimulating neural pathways that run from the prefrontal cortical surface to the cingulate gyrus, rather than hitting the target directly. It is preferable to stimulate deep structures such as the cingulate gyrus directly, but when targeting deep neural structures with rTMS, care must be taken to avoid over-stimulating superficial structures to eliminate undesired side effects such as seizures or producing unintended neural-stimulation results. It is thus necessary to avoid having too many successive pulses from the same electromagnet passing through such superficial structures while targeting the deep structure.
To effectively elicit an action potential in a neural structure, adequate stimulation must be received in a time period which is less than the minimum time (usually expressed as chronaxie) that it takes the target neural membrane to re-polarize. Otherwise threshold for generating an action potential will not be achieved. With respect to another time scale, for a given neural structure, stimulating pulses must be received within a maximum effective time interval such that the effect of the generated action potentials is additive. Neural elements are typically highly interconnected and the actual final target element to be stimulated will receive inputs from multiple sources
The pulse-rate frequency from any given electromagnet location is preferably limited, typically to a rate of less than 50 pulses per second (i.e., 50 Hz). While limiting the frequency from a single stimulating location will protect structures superficial to the deeper target, it may be impossible to effectively stimulate a deep target because of the rapid fall off of the magnetic field (roughly 1/(distance)2 at short distances). Thus, different trajectories must be stimulated in turn. We have previously suggested accomplishing this by either moving the electromagnets, as is described in Schneider and Mishelevich, U.S. patent application Ser. No. 10/821,807 (“Robotic apparatus for targeting and producing deep, focused transcranial magnetic stimulation”) or by sequentially firing electromagnets located at distributed locations. The approach of this latter case may avoid over-stimulating superficial neural structures at a single location and causing seizures or other undesired impacts, but to be successful, the pulsed magnetic fields must reach the target at a higher effective rate of stimulation than the pulsed magnetic fields hitting superficial tissue. The coordination of the orientation, timing, frequencies and power levels for controlling multiple magnets to stimulate one or more targets, and particularly deep tissue targets is a difficult task that has not yet been effectively accomplished. Described herein are methods, devices and systems for accomplishing this.
Furthermore, different tissues may have differing requirements in terms of the amount of function augmentation or suppression that they require, or that they can tolerate. For example, when seeking to suppress the activity of a remote target using slow rate rTMS delivered from multiple intersecting pathways, one or more intermediate tissues may be inadvertently suppressed in the process, when, in fact, such tissue(s) require functional augmentation.
For example, Isenberg et al., and others, have shown that either fast rate (e.g., 10 Hz) rTMS applied to the left dorsolateral prefrontal cortex (LDLPFC) or slow rate rTMS (e.g., 1 Hz) applied to the right dorsolateral prefrontal cortex (RDLPFC), are effective treatments for depression. Published studies have involved treating either of those two targets. The practical limitations of currently available equipment prevent the alternative or concurrent slow right and fast left treatment. These limitations stem from logistical difficulties in positioning TMS coils, and applying selected pulse parameters at the correct positions.
Arrays of multiple magnetic coils have been proposed. For example, Ruohonen et al. (1998) modeled in software an array of small adjacent magnets intended to stimulate the outer cortical surface of the brain. While power requirements are calculated in this study, no specific means for delivering or switching that power are disclosed. Ruohonen et al. (1999) modeled in software a multi-coil array for the purpose of limb rehabilitation. Again, no specific means for switching or delivering power to the appropriate coil were described. Instead, “the multichannel design allows the stimulus to be moved without moving the coils. This is accomplished by individually adjusting the strength and direction of the current in each coil.” Han et al. (2004) proposed a multiple coil array, but they also had no particular strategy for powering the coils other than turning them on simultaneously, and describes, “[i]n the multichannel magnetic stimulation, it is assumed that the predetermined optimal currents are fed in-phase to the coils. Therefore, all the channels are generally ON state when a stimulation pulse is applied to the subject.”
Thus, there is a need for appropriately controlling the stimulation from magnets so that the stimulation can be focused on deep tissue without creating undesired stimulation or inhibition effects in tissue superficial to the deep target. Furthermore, if effects are to be induced upon the intervening neural structures, those effects should be calculated, controllable, desirable effects. The control of the system must allow powering of the array of magnets by tapping the stored charge from one or more sources, and delivering them precisely, under the appropriate circumstances to each coil, individually. There is also a need for a system by which the pulse rate, power and pattern of stimuli delivered through intermediately juxtaposed brain tissue may be different in the different coils of the array, thereby better suiting the characteristics and therapeutic needs of that intermediate brain tissue as well as that of the principal target. Depending on the number of electromagnets, the capacity of the power sources, and other factors, it may be more appropriate to supply power for the triggering of the electromagnets from either a single power source or multiple power sources. Systems, devices and methods to address these needs, as well as others, are described in greater detail below.